Image – De-energized Motor testing demo using the AT 7 in progress at the Tableware’s Vacuum Pump 2’s Motor.
Exploring Hidden Potentials to save Energy and Unplanned Downtime
De-energized motor testing refers to the inspection and analysis of an electric motor while it is disconnected from its power source. This type of testing is crucial for assessing the motor’s condition without the risks associated with live electrical circuits. It helps in detecting issues such as insulation failures, winding defects, and mechanical problems, which can be addressed before the motor is re-energized and put back into operation.
While Resistance, Inductance, Conductance and Impedance gives us an overview of the motor’s health, these are better indicators of the insulation condition of the winding –
- Phase angle (Fi): The phase angle measures the time delay between two or more events within the same period. The complete cycle is 360 degrees. If it takes one second to complete a cycle (the period of the cycle), and one event lags the other by half a second (half a cycle, or 180 degrees), the Fi is 180 degrees. Frequency is the inverse of time (1/T), so all events with the same period occur at the same frequency. If the cycles don’t start simultaneously, one will lead or lag. Resistive, inductive and capacitive circuits will differ in how current and voltage lead or lag each other. So, when the chemical makeup of the insulation surrounding the conductors making up the coils begins to change, the Fi will change before Z, L, R or C. The Fi measurement is a leading indicator of insulation breakdown.
- Current frequency response (I/F): Inductors store magnetic fields to oppose a change in current, while capacitors store electrical charges to oppose a change in voltage. If these properties change, so does the coil or winding’s ability to store a charge or magnetic field. Insulation surrounds the conductors in the phase windings’ coils. If the insulations surrounding all the coils all have the same condition, each phase has the same storage ability. Once the insulation begins to degrade, this ability changes, creating an imbalance in the phase coils’ ability to store a magnetic field or electrical charge. The I/F response measures a coil’s ability to store a magnetic field or an electrical charge. Imbalances over 2% of I/F of any coil from the average of all phases indicate a developing fault in the winding.
Figure 1 – A snapshot of the MCA report.
There are many tools available to perform quality preventive maintenance of individual motors. Of these, motor circuit analysis (MCA) systems hold great promise for identifying motor problems before expensive failure and for improving the general efficiency of motor systems in general.
Motor circuit analysis allows the analyst to detect winding faults and rotor faults in the electric motor. One power of this type of test method is that it requires the equipment to be de-energized, which allows for initial incoming testing of the electric motors and troubleshooting when equipment fails. Primary energy losses that can be detected include phase unbalance and I²R losses, while faults include shorted windings, loose connections, ground faults and rotor faults.
A resistive fault gives off heat, as a loss. For instance, a 0.5 Ohm loose connection on a 75-kW electric motor operating at 95 amps:
- Equation 1: Resistive Losses
Kilowatts loss = (I²R)/1000 = (95² x 0.5)/1000 = 4.5 kW (demand loss)
- Equation 2: Energy Usage Loss
₹/yr = kW x hours/yr x ₹/kWh
= 4.5 kW x 8000 hours/yr x ₹6.5/kWh
= ₹ 2,34,000 / year
Electric motor phase unbalanced (inductance and impedance) affect the current imbalances, cause motors to run hotter and reduce the motor s ability to produce torque. The percentage unbalance of impedance can be evaluated to determine efficiency reduction and additional heating of the electric motor. A general rule is that, for every 10 ºC increase in operating temperature, the life of the equipment is reduced by half.
Figure 2 – Efficiency Reduction Due to Impedance Unbalance
For instance, the paperboard company has a 75-kW electric motor, that would normally be 95% efficient, that has a 3.5% impedance unbalance. The efficiency would be reduced by 4 points of efficiency, or to 91%.
Equation 3: Energy Cost Due to Phase Unbalance Losses
₹/yr savings = kW x %load x ₹/kWh x hours of operation ((100/Le) (100/He))
= 75 kW x .75 load x ₹6.5/kWh x 8000 hours ((100/91) (100/95))
= ₹ 1,36,421/ year
Figure 3 – Increase in Temperature Rise Due to Phase Unbalance
The impedance unbalance will also cause an increase in operating temperature based upon an increase in I²R losses.
In the case of the 75-kW electric motor, this means a temperature rise of about 30 ºC, or a reduction in motor insulation life to 13% of its original.
Detection using Vibration Analysis –
The Impedance Imbalance is often seen in the Vibration Analysis Spectra as Twice Line Frequency being 100 Hz or 120 Hz depending on the supply frequency. This frequency is produced as the magnetic out of balance pulls one way in the positive sine wave voltage, then negative on the opposite portion of the sine wave.
Motor Circuit Analysis is also used to evaluate the windings for contamination. Frequent cleaning of a motor’s intake (if any) and cooling fins is especially important in dirty environments Tests confirm that even severe duty, generously rated, and oversized motors can quickly fail in such conditions if they become thickly coated or if lightly coated and with their airflow reduced by half. Their insulation life can then fall to 13 25% of normal. The same phenomenon occurs if the windings become coated in contaminants.
The MCA rotor test requires inductance and impedance readings through 360 degrees of rotation of the rotor. The readings are graphed and viewed with symmetry. Rotor test results provide a definitive condition of the rotor and is often performed following identification of a possible rotor fault by vibration, as part of an acceptance program, during repair or when the motor is identified as having torque problems.
Conclusion
De-energized motor testing is a vital practice for ensuring the reliability and safety of electric motors. By employing various testing methods, technicians can detect and address potential issues before they escalate, thereby maintaining optimal motor performance and preventing unplanned downtime. The implementation of an electric motor maintenance program will have a significant impact on a company’s bottom line. Whether the company has a few hundred motors or many thousands, the simple payback from the investment into vibration and MCA is usually termed in months.
The application of these two technologies complements each other while also evaluating the progress of the maintenance program and improving upon equipment availability. Vibration analysis evaluates the mechanical condition of equipment while MCA evaluates the electrical condition of equipment. Combined, the analyst can view the complete condition of the electric motor and prevent unplanned downtime due to hidden reasons which are not possible to detect without MCA. Thereby, also optimizing our Energy usages.
The device which we plan to procure is the powerful AT 34 along with the Motor Genie. The AT-34, when powered along with the Motor Genie would help us identify every bit of the Motor’s winding giving us the opportunity to test the Motor’s health both statically and dynamically when de-energized.
